script_bindings: Check for null pointer before dereferencing proxy handler custom data (#36869)

While the vast majority of DOM proxy objects created have a non-null
pointer in the handler's extra data field, there is one place we create
a proxy object that has a null pointer:
8b05b7449d/components/script/window_named_properties.rs (L76)
. Before #36818, dereferencing this null pointer was undefined behaviour
that was silently being ignored; now that Rust 1.86 adds debug pointer
validity checks, we get a panic when trying to dereference it.

Testing: Tested about:memory with rustc 1.86.

---------

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

components/script_bindings/conversions.rs

@@ -245,6 +245,9 @@ pub unsafe fn get_dom_class(obj: *mut JSObject) -> Result<&'static DOMClass, ()>
     if is_dom_proxy(obj) {
         trace!("proxy dom object");
         let dom_class: *const DOMClass = GetProxyHandlerExtra(obj) as *const DOMClass;
+        if dom_class.is_null() {
+            return Err(());
+        }
         return Ok(&*dom_class);
     }
     trace!("not a dom object");